Hops (Humulus lupulus L.) have been used for centuries as an essential raw material in beer-brewing, providing bitterness and flavor to beer. In the last few years, the plant has gained increasing attention as a source of prenylflavonoids, a flavonoid subclass containing an apolar prenyl-side chain attached to one of the phenolic rings. These are present in the lupulin glands, found at the base of the bracteoles in the hop cones of the female plant. Of these prenylflavonoids two chalcones (xanthohumol (X) and desmethylxanthohumol (DMX)) and three flavanones (isoxanthohumol (IX), 8-prenylnaringenin (8-PN) and 6-prenylnaringenin (6-PN)) (FIG. 2), now receive much attention because of their potential health-promoting properties. X has been identified as a strong cancer chemopreventive agent, while 8-PN has been shown to be one of the most potent phytoestrogens identified so far, with considerably higher activity than the well-known soy phytoestrogens. 8-PN has been demonstrated to display in vivo estrogenic activity, to prevent bone loss in rats, to inhibit angiogenesis and metastasis and has been shown to exhibit anti-androgenic activity.
X is present as a predominant prenylchalcone in the female hop cones in concentrations up to 1% (w/w), whereas DMX is present in lower concentrations (De Keukeleire et al. (2003) J. of Agric. and Food Chem. 51, 4436-4441). The X/DMX ratio differs between the hop varieties. By isomerisation, X is transformed into IX and DMX is converted into 8-PN and 6-PN.
The estrogenic effects of hops have been recognized for decades. Hop baths have been used for the treatment of gynecological disorders and menstrual disturbances among female hoppickers were reportedly common. In 1999, Milligan et al. [J. Clin. Endocrinol. Metab. 84, 2249-2252], identified a novel phytoestrogen in hop, 8-prenylnaringenin. Although it is much weaker than 17β-estradiol (<1%), it is one of the most potent phytoestrogens identified so far, with a considerably higher activity than other phytoestrogens such as the soy-derived compounds genistein and daidzein.
It is being questioned whether dietary and/or environmental exposure to phytoestrogens could impose health risks such as endocrine disruption. In case of hop prenylflavonoids, beer is the main dietary source. The average beer consumption in the United States was calculated at about 225 ml of beer per capita per day in 2001 (USDA, 2003). When assumed that this amount was consumed as US major brand lager/pilsner beers (500-1000 μg prenylflavonoids/l beer), the daily intake of prenylflavonoids would be about 0.14 mg. However, the concentrations detected in beer (and therefore average intake) strongly depend on the brewing process, as strong ales contain up to 4 mg prenylflavonoids/l. Although X is the predominant prenylflavonoid present in hop (0.1-1% of dry weight), most of it is transformed into IX by thermal isomerisation during worth boiling. Therefore IX is the major prenylflavonoid found in beer and is present in concentrations from 500 μg/l (lager/pilsner) up to 4 mg/l (strong ale). Similarly, DMX is converted into 8-PN resulting in final concentrations in beer of up to 100 μg 8-PN/L. But despite the high activity of 8-PN, the total estrogenic activity in beer is still 500 to 1000 times lower than the concentration needed for harmful in-vivo activity (˜100 mg/l) (Milligan et al. (2002) Reproduction 123, 235-242). Moreover, many beers are now made using hop extracts instead of whole hops, giving lower concentrations of 8-PN or no 8-PN at all. Therefore, it is generally agreed that based on current knowledge, no detrimental health effects can be attributed to phytoestrogens upon moderate beer consumption.
On the other hand, many data now correlate intentional phytoestrogen intake with possible health benefits (Magee & Rowland (2004) Br. J. Nutr. 91, 513-531). Besides beer, hop based dietary supplements are marketed, claiming effects as breast enhancement and reduction of hot flushes. Overall health effects of phytoestrogens potentially result from the action of a combination of many individual phytochemicals with multiple and perhaps additive or interfering activities. Up to now, only isoflavones and lignans are considered relevant phytoestrogens in the human diet, especially because 8-PN concentrations in beer are considered to be too low for positive or negative health effects.
Several patent publications describe beneficial health effects of dietary flavonoids, for example the use of IX to prevent bone density lowering (WO04089359), the use of hop extracts in medicaments having estrogenic properties (WO02085393), and the use of IX or X in food products claiming anti-inflammatory or anti-aging properties (patent WO03090555). Moreover, the use of 8-PN in cosmetics for skin treatment (CA2426467) has also been suggested.
In order to exert in-vivo effects claimed in vitro, dietary flavonoids need to be absorbed from the gut and reach their targets unchanged. In general, monomeric flavonoids pass unmodified through the stomach into the small intestine, where absorption from the gut in the mesenteric circulation can take place. In-vitro studies indicated extensive liver biotransformation of X (Yilmazer et al. (2001a) FEBS Lett. 491, 252-256) and 8-PN (Nikolic et al. (2004) Drug Metabolism and Disposition 32, 272-279) upon absorption. However, the extent of dietary polyphenol absorption in the small intestine is rather limited (10-20%), thereby implying that a large proportion of the flavonoids reaches the colon. Naringenin, a non-prenylated analogue of 8-PN, showed intensive microbial biotransformation in the intestine, including ring cleavage and dehydroxylation (Rechner et al. (2004) Free Radic. Biol. Med. 36, 212-225), followed by absorption and urinary excretion. Little is known about intestinal transformations of prenylflavonoids. Nookandeh et al. (2004) Phytochemistry 65, 561-570, dosed 1000 mg/kg body weight of X to rats and isolated 22 metabolites from the feces. The majority (89%) of the recovered flavonoids, however, was unchanged X. The remaining fraction consisted of small amounts of different metabolites, including some IX. Avula et al. (2004) [J. Chromatogr. Sci. 42:378-382], performed a similar experiment with rats and detected mainly unchanged X next to a number of unidentified metabolites.
The possibility that IX would act as a pro-estrogen was considered by Coldham et al. (2002) Food Addit. Contam. 19:1138-1147. The assumption was based on the extensive biotransformation capacity of the liver, which includes demethylation. However, the exposure of IX to liver microsomes did not lead to an increase in estrogenic activity, from which it was concluded that no 8-PN was produced. In contrast, Nicolic et al. describe that liver microsomes can demethylate IX, but not X (Nikolic et al. (2005) J. of Mass Spectrom. 40, 289-299). However, it was shown that, besides demethylation, microsomes also modify the prenyl side-chain, finally resulting in a large variety of minor degradation products. Schaefer et al. (2003) (J. Steroid Biochem. Mol. Biol. 84, 359-360), identified low levels of 8-PN in urine after oral intake of IX by two test persons and attributed this to demethylation by the liver.
Besides the liver, the colon is also an important transformation site in the human body. The human colon contains ˜1012 microorganisms/cm3 (about 400 different species), with an enormous catalytic and hydrolytic potential. The importance of this microbial community in the metabolism of phytoestrogens in general has been clearly established. Wang et al. (2000) Chem. Pharm. Bull. 48, 1606-1610, identified two bacteria responsible for the transformation of lignans and Decroos et al. (2005) Arch. Microbiol. 183, 45-55, recently isolated a microbial consortium capable of transforming the soy phytoestrogen daidzein into equol]. Moreover, several intestinal bacteria were shown to enhance the bioavailability of phytoestrogens as they possess β-glucosidases, which are necessary for the hydrolysis of phytoestrogen glycosides (Rowland et al. (2003) Br. J. Nutr 89, s45-S58). Thus, the gut microbiota are considered to be a factor of importance for phytoestrogen bioavailability (Turner et al. (2003) Nutr. Rev. 61, 204-213).
As only the essential oil and the alpha-acids present in the female hop cones are of economic interest as important brewery ingredients, the different extraction methods of hop which have been developed aim to specifically extract only these compounds. On the one hand, CO2 is currently the most accepted solvent for the manufacture of hop extracts (Palmer & Ting (1995) Food Chem. 52, 345-352). In comparison with the procedures that use conventional organic solvents (ethanol, hexane, methanol, or methylene chloride), CO2-extraction provides more selective extracts that can be used for the production of beers as a good alternative for whole hops or hop pellets. CO2-extracts form the basis of a large number of further derived and purified products, such as iso-alpha-acids and reduced derivatives. Another procedure for further purification of CO2-extract, by removal of unwanted prenylflavonoids, is disclosed in U.S. Pat. No. 3,794,744.
On the other hand, different procedures have been developed to specifically recover and purify prenylflavonoids (mainly X). Examples of these extraction methods are disclosed in U.S. Pat. No. 4,121,040 and German patent DE19939350. As xanthohumol can easily be recovered using these processes, little interest has been shown in developing a procedure to chemically synthesize X. 8-PN, however, is more difficult to recover from natural extracts because of the low concentrations present in the hop cone. Therefore, synthesis strategies have been developed to produce 8-PN by prenylation of the commercially available naringenin. First, 8-PN is produced by the low yielding unselective direct C-prenylation of naringenin or starting from phloroacetophenone. Efficient small scale chemical synthesis was achieved by europium(III)-catalyzed Claisen rearrangement (Gester et al. (2001) Tetrahedron 57, 1015-1018). Recently, industrial scale production based on this method has been described in the European patent EP1524269.
Despite the widespread industrial use of hop and hop extracts, there is no efficient method for the production of bioactive prenylated phytoestrogens such as 8-PN from a natural source.